Research and observation suggest that some trees or logs produce mostly straight lumber, while others result in a larger proportion of warped pieces. The range of lumber warp variability among logs has been found to be especially broad among butt logs, a class of logs which also generally includes those with the greatest log-average lumber crook and bow. To illustrate, FIG. 1 shows data from lumber cut from 30 pine trees harvested in Georgia, and compares log-average crook values for logs from three different height locations in each tree—butt, second, and third.
In general, butt logs are the most affected by lumber crook. In fact, about one-third of these trees (9 of 30) had butt logs with substantially greater log-average crook than any of the other logs. The other two-thirds of the butt logs had somewhat greater log-average crook than that of the second or third logs. The log-average bow values are compared by log position in the tree in FIG. 2. The same observations that were made for crook also apply to bow, although there are perhaps relatively fewer trees having butt logs with extreme log-average values, and the difference between those extreme values and the log-average bow of the other logs is somewhat less than in the case of crook.
These Figures suggest that for crook and bow, the most warp-prone logs are usually found among a minority of the butt logs. One means of partially distinguishing between warp-prone and warp-stable logs is by using the average stress-wave velocity of the log, as measured for example, using resonance methods. FIGS. 3 and 4 show how log-average crook and bow, respectively, relate to average log stress-wave velocity in loblolly pine butt logs harvested in Arkansas. Logs with stress-wave velocity at or near the high end of the range have relatively low log-average crook and bow. Those logs with lower stress-wave velocities, which constitute the majority of the logs, may also have low log-average crook and bow. However, a fraction of the lower-stress-wave velocity logs have high log-average warp. In other words, high-stress-wave velocity logs have low potential for lumber warp, but low-stress-wave velocity logs are not necessarily highly warp-prone. Consequently, for the majority of logs (those which are not near the high end of the range of stress-wave velocity), the average stress-wave velocity of the log is not in itself an effective means to discriminate between logs with high potential for lumber warp and those with low potential.
Accordingly, a need exists for a method to detect warp potential of lumber to be derived from a raw material, such as a log or stem, and to reduce that warp potential before the lumber is derived.